It has long been recognized that acute infections evoke a great variety of nonspecific reactions that may be used to monitor the clinical course of patients. Among these are fever, changes in white blood cell count and amount of C-reactive protein (CRP) in circulation. It has only recently been realized that there may be a single key mediator for all three, interleukin 1. This is a protein induced in and released from mononuclear macrophages by microorganisms and other inflammation-producing substances. In this review I present the current understanding of what interleukin 1 is, what it can do and how it may bring about changes in fever patterns, in total and differential white blood cell counts and in levels of CRP. In addition a number of recent reports are given which reaffirm the usefulness of sequentially monitoring these nonspecific reactions for evaluating the effectiveness of treatment.